Ignore dead_code lint on AssertSync trait

```
error: trait `AssertSync` is never used
   --> src/lib.rs:226:15
    |
226 |         trait AssertSync: Sync {}
    |               ^^^^^^^^^^
    |
    = note: `-D dead-code` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(dead_code)]`
```

src/lib.rs

@@ -223,6 +223,7 @@ impl AtomicWaker {
     /// Create an `AtomicWaker`.
     pub const fn new() -> Self {
         // Make sure that task is Sync
+        #[allow(dead_code)]
         trait AssertSync: Sync {}
         impl AssertSync for Waker {}